What is the Metallic Shield of the high voltage power cables

The metallic portion of the insulation shield or screen is necessary to provide a low resistance path for charging current to flow to ground. It is important to realize that the extruded shield materials will not survive a sustained current flow of more than a few
milliamperes. These materials are capable of handing the small amounts of charging current, but cannot tolerate unbalanced or fault currents.

The metallic component of the insulation shield system must be able to accommodate these higher currents. On the other hand, an excessive amount of metal in the shield of a single-conductor cable is costly in two ways. First, additional metal over the amount that
is actually required increases the initial cost of the cable. Secondly, the greater the metal component of the insulation shield, the higher the shield losses that result from the flowof current in the central conductor. This subject is treated more completely in Chapter 13,
Ampacity.

A sufficient amount of metal must be provided in the cable design to ensure that the cable will activate the back-up protection in the event of any cable fault over the life of that cable. There is also the concern for shield losses.

It therefore becomes essential that:
□ The type of circuit interrupting equipment to be analyzed. What is the design and
operational setting of the fuse, recloser, or circuit breaker?
□ What fault current will the cable encounter over its life?
□ What shield losses can be tolerated? How many times is the shield to be grounded?

Will there be shield breaks to prevent circulating currents? Although there are constructions such as full and one-third neutral listed in ICEA standards for single-conductor, URD and UD cables, these may not be the designs that are the most economical for a particular installation. Studies have been published on the optimum amount of metal to use in the neutral [7-3,7-4]. Documents such as these should be reviewed prior to the development of a cable design. In Chapter 13, Ampacity, there is an in-depth discussion of shield losses.

source : Electrical Power Cable Engineering book